1. Field of the Invention
The present invention relates to a fixed type constant velocity joint in which a rotary shaft on the driving side and a rotary shaft on the driven side are connected together and which is capable of transmitting torque at constant angular velocities even when the two shafts form an angle. It is capable of angular displacement alone without plunging, and is utilized in automobiles and various industrial machines.
2. Brief Description of the Prior Art
While the connecting construction of the drive shaft of an automobile differs according to the vehicle suspension system, a differential gear, or a final reduction gear, is attached to the vehicle body side, for example, in a vehicle employing an independent suspension system, and the opposite ends of the drive shaft are connected to the differential and the axle, respectively, through universal joints. And in order to allow displacement of the drive shaft in response to the movement of the suspension, the construction is such as to allow angular displacement of the drive shaft in the wheel-side connection and angular displacement and axial displacement in the vehicle body-side connection.
As for the above-mentioned universal joints, constant velocity joints are frequently used at present, and the wheel-side connection uses the fixed type constant velocity joints which allow only the angular displacement of the two shafts, such as the Rzeppa type, while the vehicle body-side connection uses the slide type constant velocity joints which allow angular displacement and axial displacement between the two shafts, such as the double offset type, tripod type, and cross group type.
A fixed type constant velocity joint shown in FIGS. 9A and 9B comprises an outer joint member 1 having six curved guide grooves 1b axially formed in a spherical inner peripheral surface 1a, an inner joint member 2 having six curved guide grooves 2b axially formed in a spherical outer peripheral surface 2a, and having spline (or serration) holes 2c, torque transmitting balls 3 disposed one by one in six ball tracks defined by cooperation between the guide grooves 1b of the outer joint member 1 and the guide grooves 2b of the inner joint member 2, and a cage 4 holding the torque transmitting balls 3.
The center of curvature of the inner peripheral surface 1a of the outer joint member 1 and the center of curvature of the outer peripheral surface 2a of the inner joint member 2 are each coincident with the joint center O. The center of curvature A of each of the guide grooves 1b of the outer joint member 1 and the center of curvature B of each of the guide grooves 2b of the inner joint member 2 are axially offset by an equal distance on opposite sides of the joint center O (in the example shown in the same figure, the center A is on the opening side of the joint and the center B is on the innermost side of the joint). Therefore, the ball tracks defined by cooperation between the guide grooves 1b and 2b are wedge-shaped, opening to one axial side (in the example shown in the same figure, to the opening side of the joint).
In the case where the two shafts make no angular displacement, that is, the axes of rotation of the two shafts form a straight line, as shown in FIG. 9A, the centers of all of the torque transmitting balls 3 are in a plane perpendicular to the axis of rotation including the joint center O. When the outer and inner joint members 1 and 2 make an angular displacement by an angle θ, the cage 4 places the torque transmitting balls 3 in a plane bisecting the angle θ, thereby securing the constant velocity property of the joint.
A conventional fixed type constant velocity joint is known to have eight torque transmitting balls in order to realize further size compaction and weight reduction while securing strength, loading capacity and durability exceeding that provided by a fixed type constant velocity joint having six torque transmitting balls as shown in FIGS. 9A and 9B.
One of the main damages to fixed type constant velocity joints during high-angle operation is a mode called cage column shear fracture caused by spherical edge cutting-in of the outer and inner joint members. FIG. 3 is an enlarged sectional view for explaining a damage mode of a fixed type constant velocity joint, showing the vicinity of a torque transmitting ball positioned outermost when a maximum operating angle is taken. As can be seen from the same view, when the spherical edge contact points (loading points) of the outer and inner joint members are greatly offset axially of the cage, the shear stress in the cage column increases, imposing an excessive moment load thereon, so that the cage strength considerably lowers.